0.7x2-4.2x-0.7=0

Solution for 0.7x2-4.2x-0.7=0 equation:

0.7x^2-4.2x-0.7=0

a = 0.7; b = -4.2; c = -0.7;
Δ = b2-4ac
Δ = -4.22-4·0.7·(-0.7)
Δ = 19.6
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-4.2)-\sqrt{19.6}}{2*0.7}=\frac{4.2-\sqrt{19.6}}{1.4}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-4.2)+\sqrt{19.6}}{2*0.7}=\frac{4.2+\sqrt{19.6}}{1.4}
$